The purpose of the centripetal air bleed device placed inside the high pressure rotor is to bring a flow of air bled from a stage of the compressor to stages of the turbine that need to be cooled. It is important for the cooling air that reaches the blading of the high pressure turbine which is subjected to high temperatures to be at a pressure which is sufficient to enable a protective film of air to be formed around the turbine blades, and for the air to be at a temperature that is as low as possible.
The bleed device may include bleed channels formed in the upstream disk, as disclosed in FR 2 609 500 and FR 2 614 654, or bleed tubes placed in the annular cavity between two disks, as disclosed in U.S. Pat. No. 5,475,313.
The flow of air bled from the stream penetrates into the annular groove via the interstice separating the inside platforms of the stator vanes from the rim of the upstream disk by traveling in a direction that is substantially axial, and it then passes through holes in the rotating shroud. It will thus be understood that the velocity of the air at the inlets to the holes relative to the rotating disk is relatively high, which gives rise to an increase in the relative total temperature of the air in the holes and to a non-negligible loss of head in said zone. This temperature increase is naturally to be found in the flow of air delivered to the blades of the turbine. The loss of head decreases the flow rate of the bleed air.